Antigen fragments for the diagnosis of toxoplasma gondii

ABSTRACT

The invention described herein relates to a method for identifying antigen fragments of  Toxoplasma gondii  proteins, and their use as diagnostic and immunogenic agents. Said method is implemented by means of selection of DNA fragments libraries of the parasite with sera of subjects who have been infected, using the phage display technique, and is characterised in that it uses the expression/exposure vector λKM4. The method allows also to identify antigen fragments related to the time of the infection.

The invention described herein relates to a method for identifying antigen fragments of Toxoplasma gondii proteins, and their use as diagnostic and immunogenic agents. Said method is implemented by means of selection of cDNA libraries of the parasite or of DNA fragments of specific genes of the parasite with sera of subjects who have been infected by the parasite, using the phage display technique, and is characterised in that it uses the vector λKM4.

The invention described herein also relates to the technical field of the preparation of diagnostic means not applied directly to the animal or human body and furnishes compounds, methods for their preparation, methods for their use and compositions containing them which are suitable for industrial application in the pharmaceutical and diagnostic field, particularly for the detection and diagnosis of Toxoplasma gondii infections, as well as for the treatment and prevention of said infections.

BACKGROUND TO THE INVENTION

Early diagnosis is a priority and highly desirable objective in all fields of medicament, particularly because it allows an appreciable improvement in the patient's life and a concomitant saving on the part of health care systems or on the part of the actual patients. In the particular case of the invention described herein, early diagnosis is that of potential or existing Toxoplasma gondii infection in pregnant women, with particular concern for the health of the foetus, and in infected subjects, particularly those with impaired immunity.

Toxoplasma gondii is an obligate intracellular parasite that infects all mammalian cells, including those of human subjects (McCabe and Remington, N. Engl. J. Med. 1988, 318-313-5), and other animal genera, e.g. birds. The life cycle of the parasite is complex and one may distinguish between three stages of infection: tachyzoite (asexual), bradyzoite (in tissue cysts, asexual) and sporozoite (in oocysts, sexual reproduction). Transmission typically occurs through ingestion of undercooked meat harbouring tissue cysts or vegetables contaminated with oocysts shed by cats. Human infection is generally asymptomatic and self-limiting in immunocompetent hosts. In contrast, in subjects with impaired immunity (particularly those affected by AIDS), toxoplasmosis is a severe opportunist infection, which may give rise to encephalitis with very serious outcomes (Luft, B. J., Remington J. S., 1992, Clin. Infect. Dis. 15, 211-22). Moreover, contracting primary infection during pregnancy may lead to miscarriages or to severe foetal disease in mammals.

For an extensive overview of the problem of toxoplasmosis the reader is referred to the specialistic medical literature.

Diagnosis of T. gondii infection is established by isolating the micro-organism in the blood or body fluids, identifying the parasite in tissues, detecting specific nucleotide sequences with PCR, or by detecting specific anti-T. gondii immunoglobulins produced by the host in response to the infection (Beaman et al., 1995 Principles and Practice of Infectious Diseases 4th Ed., Churchill Livingstone Inc., New York, 2455-75; Remington J S et al. 1995, Infectious Diseases of the Fetus and Newborn Infant, W. B. Saunders, Philadelphia, Pa., 140-267).

One of the main problems in diagnosing T. gondii infections has to do with pregnant women. To implement suitable therapies in good time and avoid possible damage to the foetus it is very important to establish if parasitic infection occurred before or after conception. This is generally done by attempting to detect the presence of the various classes of anti-Toxoplasma immunoglobulins (IgG, IgM, IgA, avidity of IgG). For this reason, the availability of specific, sensitive diagnostic agents is desirable.

T. gondii antigens have long been known and available, first of all as antigen mixtures obtained in various ways (FR 2,226,468, Mérieux; SU 533376, Veterinary Research Institute; JP 54044016, Nihon Toketsu Kanso), then as subsequent isolations of pure antigens (EP 0 082 745, Mérieux; EP 0 301 961, INSERM, Pasteur; WO 89/5658, Transgene) and their characterisation both as proteins, and of their respective genes (WO 89/08700, U. Leland, Dartmouth Coll.; U.S. Pat. No. 4,877,726, Res. Inst. Palo Alto; WO 89/12683, INSERM, Pasteur; EP 0 391 319, Mochida Pharm.; IT 1,196,817, CNR; EP 0 431 541, Behringwerke; WO 92/01067, CNRS; WO 92/02624, U. Flinders; WO 92/11366, Innogenetics, Smithkline Beecham; U.S. Pat. No. 5,215,917, Res. Inst. Palo Alto; WO 92/25689, FR 2702491, INSERM, Pasteur; WO 96/02654, bioMeriéux, Transgene; EP 0 710 724 Akzo; EP 0 724 016, bioMeriéux; EP 0 751 147, Behringwerke; U.S. Pat. No. 5,633,139, Res. Inst. Palo Alto; WO 97/27300, Innogenetics; U.S. Pat. No. 5,665,542, U.S. Pat. No. 5,686,575, Res. Inst. Palo Alto; WO 99/32633, Heska; JP 11225783, Yano; WO 99/61906, Abbott; WO 99/66043, Smithkline Beecham; JP 2000300278, Yano; WO 00/164,243, Virsol).

Numerous studies have found various different antigenic proteins of T. gondii and the gene sequences of these have also been determined.

Among the most interesting proteins both for diagnostic and therapeutic purposes, in the form of vaccines, we should mention: the surface antigens SAG1 (or P30) (WO 89/08700, Stanford University; WO 89/12683 Pasteur, INSERM; WO 94/17813, WO 96/02654, Transgene, bioMeriéux; EP 0 724 016, WO 99/61906, U.S. Pat. No. 5,962,654, Harning et al., Clinical and Diagnostic Laboratory Immunology, May 1996, 355-357); SAG2 (or P22) (Parmley et al., 1992, J. Clin. Microbiol. 30, 1127-33); the dense granule proteins GRA1 (or P24) (EP 0 301 961, Pasteur, INSERM; WO 89/05658, Transgene, Cesbron-Delauw, et al., 1989 P.N.A.S. USA 86, 7537-41); GRA2 (or P28) (WO 93/25689, INSERM, Pasteur; U.S. Pat. No. 5,633,139, U.S. Pat. No. 5,665,542, U.S. Pat. No. 5,686,575, Res. Inst. Palo Alto; Prince et al., Mol. Biochem. Parasitol., 34 3-14); GRA4 (Mevelec et al., Mol. Biochem. Parasitol. 56, 227-38); GRA6 (or P32) (FR 2,702,491, INSERM, Pasteur; Lecordier al., Mol. Biochem. Parasitol. 70, 85-94); GRA7 (WO 99/61906, Abbott; Jacobs et al., Mol. Biochem. Parasitol. 91, 237-49); GRA3 (Robben et al. 2002, J. Biol. Chem. 277, 17544-47): the rhoptry antigens ROP1 (or P66) (U.S. Pat. No. 5,976,553, U. Leland; EP 0 431 541, Innogenetics); ROP2 (or P54) (Sharma et al., J. Immunol., 131, 377-83).

As described in the above-mentioned references, the antigens were obtained with well-known recombinant cDNA techniques in expression vectors. For example, for the antigen SAG1, WO 98/08700 uses known expression vectors in phage λgt11. WO 98/12683 uses the same phage and transfects E. coli with a proprietary plasmid, or by preparing a special expression cassette, as in WO 96/02654. EP 0 724 016 obtains mimotypes, using combinatorial expression libraries of peptides. EP 0 301 961 describes how to obtain excretion-secretion antigens with molecular weights ranging from 20 kDa to 185 kDa. WO 89/05658 describes a protein containing the epitopes of the 24 kDa protein recognised by the antibodies produced against Toxoplasma excretion-secretion antigens; this protein is obtained by transfection of cells by means of expression vectors. The antigen P28 (GRA2) is described in U.S. Pat. No. 5,633,139 and the method of obtaining it is again implemented through expression in phage λgt11. The antigen P32 (GRA6) is described in patent FR 2,702,491, the antigen ROP1 (P66) in U.S. Pat. No. 5,976,553, P35 (or GRA8) in EP 0 431 541, WO 99/57295 and WO 99/61906, and lastly P68 in EP 0 431 541.

It should be stressed that all these antigens are obtained by means of molecular biology techniques that use the expression of proteins in bacterial cells. None of the documents cited describe the technique of expression/exposure of libraries of cDNA deriving from Toxoplasma gondii in the lambda phage (phage display) to obtain fragments of antigens of the pathogen.

The invention described herein uses a new vector of DNA expression and protein exposure as molecular fusion with the amino-terminal part of protein D of the lambda bacteriophage capsid (pD) (λKM4).

The expression/exposure vector was described for the first time in patent application PCT/IT01/00405, filed on 26 Jul. 2001, the most important part of which is incorporated herein. This vector, called λKM4, differs from that used in expression only experiments (λgt11) in that the recombinant protein coded for by the DNA fragment is expressed as fusion with a protein of the bacteriophage itself and then exposed on the capsid. According to the vector project, the phage exposes the protein fragment on the surface only if its open reading frame (ORF) coincides with pD. The size of the fragments of DNA cloned in our libraries was selected in order to represent a population of medium size ranging from 300 to 1000 nucleotide base pairs (bp), and, for statistical reasons, most of the out-of-frame sequences contain stop codons which do not permit their translation and consequently exposure on the surface of the phage.

SUMMARY OF THE INVENTION

It has now been found that the combination of the affinity selection and phage display techniques, together with the use of the vector KM4, provides a method for the identification of specific antigen fragments of Toxoplasma gondii by means of the selection of display libraries of DNA fragments with sera of infected individuals. DNA fragments are obtained either from cDNA of whole parasite or from DNA encoding for known specific gene products. With this method it proves possible to identify antigen fragments from very large libraries (i.e. expressing a large number of different sequences). The antigen fragments thus identified enable specific ligands to be obtained, which in turn can be used as diagnostic and therapeutic means.

Therefore, one object of the invention described herein is a method for the identification of antigen fragments of Toxoplasma gondii proteins, by means of the selection of libraries of DNA fragments with sera of subjects who have been infected by the parasite, using the phage display technique, characterised in that it uses the vector λKM4.

Another object of the present invention are antigen fragments obtainable with the above-mentioned method, both isolated and characterised, and as sets of antigen fragments called “collections”. The invention described herein also extends to the antigen portions of said fragments (epitopes).

The use of said antigen fragments as diagnostic agents and the related diagnostic aids containing them, for example in the form of kits or other supports, constitute a further object of the present invention.

The use of said antigen fragments as active agents, particularly with an immunogenic action, for the preparation of medicaments for the prevention and therapy of Toxoplasma gondii infection, constitute a further object of the present invention.

Another object of the present invention are the gene sequences coding for the above-mentioned antigen fragments, their use as medicaments, particularly for the prevention and therapy of Toxoplasma gondii infection, e.g. as gene therapy. The present invention also extends to the gene sequences that hybridise with the sequences of the above-mentioned fragments in stringent hybridisation conditions.

Another object of the present invention are anti-epitope antibodies and their use in the preparation of diagnostic, preventive and therapeutic means, e.g. as conjugates with active ingredients such as chemo-therapy agents. Antibodies can be generated also against collections of said epitopes.

The method provided by the present invention makes it possible to confirm the use of the Toxoplasma gondii antigens described above as such as diagnostic agents and also to identify in known antigens the epitopes that trigger an immune response in human subjects; this portion is a further object of the present invention; but it also makes it possible to identify the antigenic function of proteins of Toxoplasma gondii, or of portions thereof, which, though their structure and possibly their physiological function may be known, are unknown as regards their antigenic function, and such function comes within the framework of the present invention; lastly, the method according to the present invention also provides new antigen fragments of Toxoplasma gondii proteins, that constitute yet another object of the present invention.

Another object of the present invention is the use of the antigen fragments thus identified for the preparation of means of diagnosing the infection, as well as the actual diagnostic means containing them. The use realtes also to the diagnosis of the time of the infection, in particular by the IgG avidity assay.

Another object of the present invention is the use of the antigen fragments thus identified as medicaments, particularly for the preparation of formulations, and particularly in the form of vaccines, which are useful for the prevention and cure of the infection. The vaccines according to the present invention are suitable for use in humans and other animals (particularly pigs, cats, sheeps).

Another object of the present invention are ligands generated from the above-mentioned antigen fragments and the related collections and the use of such ligands for the preparation of diagnostic means for the detection of the infection, with particular reference to the time of infection, as well as therapeutic means for the prevention and treatment of the infection itself.

Another object of the present invention is a method for the diagnosis of Toxoplasma gondii infection, comprising the selection of sera of subjects affected or suspected of being affected by said infection with the above mentioned antigen fragments and/or their collection and/or at least one ligand and/or antibody.

These and other objects will be illustrated here below in detail, also by means of examples and figures, where FIG. 1 represents the map of the vector λKM4.

DETAILED DESCRIPTION OF THE INVENTION

The present invention comprises the construction of expression/exposure libraries of DNA fragments prepared from Toxoplasma gondii cells, the selection of such libraries with the sera of patients who have been infected by Toxoplasma gondii, the characterisation of the antigen fragments, and the use of said fragments for developing selective diagnostic means.

Optionally, the present invention may entail the generation of specific ligands for said antigen fragments (e.g. human recombinant antibodies or humanised murine recombinant antibodies) and the construction of selective diagnostic means that incorporate the ligands generated.

Antibodies and ligands of the present invention can be obtained according according to the general common knowledge and conventional methods.

The method according to the present invention advantageously combines affinity selection and the power of phage display.

What is meant by phage display, as understood by the person of ordinary skill in the art, is a strategy based on the selection of expression/exposure libraries in which small protein domains are exposed on the surface of bacteriophages containing the corresponding genetic information.

The method implemented according to the present invention for the first time provides new and advantageous analysis possibilities:

-   -   the use of small amounts of serum to identify antigen fragments         of the infectious agent,     -   the possibility of selecting only the domains responsible for         the interaction with the antibodies, without having to express         the entire gene, the product of which may be insoluble or toxic;     -   lastly, the possibility of effecting successive cycles of         selection using sera from different patients or mixtures of sera         facilitates the identification of cross-reactive antigens which         represent one of the main objectives of the present invention.

For the above-mentioned reasons, for each library, messenger RNA was purified from an adequate number of cells (e.g. 10⁶ cells), using common commercially available means, from which the corresponding cDNA was generated. The latter was fragmented (by means of the bacterial enzyme DNaseI) and then cloned in the expression/exposure vector λKM4 (see example).

In the other embodiment, relating to specific T. gondii gene, each specific T. gondii gene, was amplified from the DNA of the parasite (either cDNA or genomic DNA, both prepared by using common commercially available kits) by means of PCR with specific synthetic oligonucleotides. DNA of single genes was then fragmented randomly by means of the bacterial enzyme DNaseI and then cloned as a pool in the expression/exposure vector λKM4.

The amplification of the libraries was done by means of normal techniques with which the expert in the field is familiar, e.g. by plating, growth, elution, purification and concentration (Sambrook et al., 1989, Molecular Cloning: a laboratory manual, Cold Spring Harbor Laboratory Press, NY). The libraries were then used to develop the selection conditions, screening and characterisation of the sequences identified. Lastly, the phage clones identified were characterised by immunoenzymatic assays.

A library of the phage display type, constructed using cDNA deriving from cells of pathogenic organisms, makes it possible to exploit affinity selection, which is based on incubation of specific sera (reactive with the pathogen) with collections of bacteriophages that express portions of proteins of the pathogen on their capside and that contain the corresponding genetic information. The bacteriophages that specifically bind the antibodies present in the serum are easily recovered, remaining bound (by the antibodies themselves) to a solid support (e.g. magnetic beads); the non-specific ones, by contrast, are washed away. Direct screening, i.e. the analysis of the ability of single phage clones to bind the antibodies of a given serum, is done only at a later stage, when the complexity of the library (i.e. the different number of sequences) is substantially reduced, precisely as a result of the selection.

The use of selection strategies allows faster analysis of a large number of different protein sequences for the purposes of identifying those that respond to a particular characteristic, e.g. interacting specifically with antibodies present in the serum of patients who have been infected by the pathogen. What is more, the combination of affinity selection and phage display makes it possible to use a smaller amount of serum for each analysis. The direct screening of a classic cDNA library, in fact, entails the use of large amounts of serum, which are not always easy to obtain. For example, to analyse a library of approximately 10⁶ independent clones it would be necessary to incubate along with the preselected serum the numerous filters containing a total of approximately 10⁷ phage plaques transferred from the different culture plates with the infected bacteria (e.g. a serum volume of 1-10 ml). The use of a display-type library, on the other hand, permits affinity selection in small volumes (0.1-1 ml) prior to direct screening, and from limited amounts of serum, such as, for example, 10 μl.

Lastly, given that a very large number of bacteriophages can be contained in small volumes (e.g. 10¹¹ phage particles are normally contained in a volume of 0.1 ml), and affinity selection is done in small volumes (0.1-1 ml), a further advantage of the use of display-type libraries consists in analysing a number of independent clones (particles of recombinant phages exposing different cDNA sequences on their surfaces) 10-100 times greater (e.g. 10⁸ different bacteriohages) than expression-alone libraries where, as a result of technical problems, not more than 10⁶ independent clones are normally analysed.

As regards industrial applicability, one possible realisation of the present invention is in the form of diagnostic kits containing the antigen fragments and/or ligands and/or antibodies described above.

The diagnostic kits which are the object of the present invention are known to the expert in the field and do not require any particular description. By way of an example, the reader is referred to the patent literature cited above, to which may be added U.S. Pat. No. 6,265,176 and WO 01/63283 as further references.

Similar considerations hold good for the therapeutic application, where the preparation of medicaments or vaccines comes within the framework of general knowledge; for further reference purposes the reader is again referred to the patent literature cited in the present description.

The invention will now be illustrated in greater detail by means of examples and figures, where FIG. 1 presents the map of the vector λKM4.

EXAMPLE 1

Construction of the Vector λKM4

This technique is described in international patent application No. PCT/IT01/00405, filed on 26.07.2001 and incorporated herein for reference purposes, explicitly mentioning the references cited therein. FIG. 1 represents the map of the vector λKM4. The plasmid pNS3785 (Sternberg and Hoess, 1995, Proc. Natl. Acad. Sci. USA., 92:1609-1613) was amplified by inverse PCR using the synthetic oligonucleotides 5′-TTTATCTAGACCCAGCCCTAGGAAGCTTCTCCTGAGTAGGACAAATCC-3′ (SEQ ID No 1) bearing the sites XbaI and AvrII (underlined) for the subsequent cloning of the lambda phage, and 5′-GGGTCTAGATAAAACGAAAGGCCCAGTCTTTC-3′ (SEQ ID No 2) bearing the site XbaI. In inverse PCR a mixture of Taq DNA polymerase and Pfu DNA polymerase was used to increase the fidelity of the DNA synthesis. Twenty-five amplification cycles were performed (95° C.-30 sec, 55° C.-30 sec, 72° C.-20 min). The autoligation of the PCR product, previously digested with XbaI endonuclease gave rise to the plasmid PKM3. The lambda gene pD was amplified with PCR from the plasmid pNS3785 using the primers 5′-CCGCCTTCCATGGGTACTAGTTTTAAATGCGGCCGCACGAGCAAAGAAACCTTTAC-3′ (SEQ ID No 3) e 5′-AGCTTCCTAGGGCTGGGTCTAG-3′ (SEQ ID No 4) containing the restriction sites NcoI, SpeI, NotI and EcoRI, respectively, (underlined). The PCR product was then purified, digested with NcoI and EcoRI endonuclease and recloned in sites NcoI and EcoRI of pKM3, resulting in the plasmid pKM4 bearing only the restriction sites SpeI and NotI at the 5′ end of the protein gpD. The plasmid was then digested with XbaI endonuclease and cloned in the XbaI site of the lambda phage Dam15imm21nin5 (Sternberg and Hoess, 1995, Proc. Natl. Acad. Sci. USA., 92:1609-1613).

Construction of cDNA Library from Tachyzoites of Toxoplasma gondii

Tachyzoites of the protozoon Toxoplasma gondii (RH strain) were grown in vitro in monkey kidney cells (“VERO” African green monkey cells) using DMEM culture medium containing 10% foetal bovine serum, 2 mM glutamine and 0.05 mg/ml gentamicin (Gibco BRL, Canada). The parasites were collected after complete lysis of the host cells and purified by filtration (filter porosity 3 μm) followed by centrifuging. 4 μg of mRNA were isolated from 10⁷ tachyzoites using the “QuickPrep Micro mRNA Purification Kit” (Amersham Pharmacia Biotech, Sweden) and following the manufacturer's instructions. The double-helix cDNA was synthesised from 200 ng of poly(A)⁺ RNA using the “SMART cDNA Library Construction Kit” (Clontech, CA, USA) and following the manufacturer's instructions. 10 μg of total cDNA were then fragmented randomly using 0.5 ng of the endonuclease DNaseI (Sigma-Aldrich, USA). The mixture of cDNA and DNaseI was incubated for 20 minutes at 15° C. and the cDNA fragments were purified with extraction in phenol/chloroform and subsequent purification by means of the “QIAquick PCR Purification Kit” (Qiagen, CA, USA), following the manufacturer's instructions. The 3 μg ends of the cDNA fragments were “flattened” by incubating the DNA with 9 units of the enzyme T4 DNA polymerase (New England Biolabs, MA, USA) for 60 minutes at 15° C. The fragments were then purified by means of extraction in phenol/chloroform and subsequent precipitation in ethanol. 500 ng of the resulting DNA were bound with a 20-fold molar excess of “synthetic adaptors” for the purposes of adding the restriction sites SpeI and NotI to the ends of the fragments. Six adaptors were used, obtained by hybridisation of the following pairs of oligonucleotides: K185 5′-CTAGTCGTGCTGGCCAGC-3′ (SEQ ID No 5) and K186 5′-GCTGGCCAGCACGA-3′ (SEQ ID No 6); K187 5′-CTAGTCGTGCTGGCCAGCT-3′(SEQ ID No 7) and K188 5′-AGCTGGCCAGCACGA-3′ (SEQ ID No 8); K189 5′-CTAGTCGTGCTGGCCAGCTG-3′ (SEQ ID No 9) and K190 5′-CAGCTGGCCAGCACGA-3′ (SEQ ID No 10); K191 5′-TCTGGTGGCGGTAGC-3′ (SEQ ID No 11) and K192 5′-GGCCGCTACCGCCACCAGA-3′ (SEQ ID No 12); K193 5′-TTCTGGTGGCGGTAGC-3′(SEQ ID No 13) and K194 5′-GGCCGCTACCGCCACCAGAA-3′ (SEQ ID No 14); K195 5′-TTTCTGGTGGCGGTAGC-3′ (SEQ ID No 15) and K196 5′-GGCCGCTACCGCCACCAGAAA-3′(SEQ ID No 16). The excess of unligated adaptors was removed from the ligation mixture by electrophoresis on 2% agarose gel and the cDNA fragments with molecular weights ranging from 300 bp to 1000 bp were excised from the gel and purified by means of the “Qiaquick gel extraction kit” (Qiagen, CA, USA) following the manufacturer's instructions. The vector % KM4 was digested with SpeI/NotI and for the construction of the library 6 ligation mixtures were performed, each containing 0.4 μg of vector and approximately 7 ng of insert. After overnight incubation at 4° C. the ligation mixtures were packaged in vitro with the “Ready-To-Go lambda packaging kit” (Amersham Pharmacia Biotech, Sweden) and plated for infection of BB4 cells (bacterial cells of E. coli strain BB4; Sambrook et al., 1989, Molecular Cloning: a laboratory manual, Cold Spring Harbor Laboratory Press, NY). After overnight incubation at 37° C. the phage was eluted from the plates with SM buffer (Sambrook et al., 1989, Molecular Cloning: a laboratory manual, Cold Spring Harbor Laboratory Press, NY), purified, concentrated and stored at −80° C. in SM buffer containing 7% dimethylsulphoxide. The complexity of the library calculated as the number of total independent clones with inserts was 10⁷ clones.

Affinity Selection

Two distinct methods were used for selecting the phage library with human sera. In the first method 100 μl of magnetic beads coated with Protein G (Dynabeads Protein-G, Dynal, Norway) were incubated with 10 μl of human serum for 30 minutes at room temperature. The beads were then incubated for 1 hour at 37° C. with blocking solution consisting of: 5% skimmed milk powder in PBS (Sambrook et al., 1989, Molecular Cloning: a laboratory manual, Cold Spring Harbor Laboratory Press, NY), 0.05% Tween 20, and MgSO₄ 10 mM. Approximately 10¹⁰ phage particles of the library were added to the beads and diluted in 1 ml of blocking solution for a further 4-hour incubation at room temperature with weak stirring. In the second method 40 μl of “M280-Tosyl activated” magnetic beads (Dynal, Norway) were coated with human anti-IgM antibodies (Sigma-Aldrich, USA) following the manufacturer's instructions. The beads were then washed with PBS/TritonX-100 1% and incubated with 10 μl of human serum in 300 μl of blocking solution for 2 hours at room temperature. After washing the beads three times with washing solution (PBS, 1% TritonX100, 10 mM MgSO₄), 1010 phage particles of the library were added to the beads and diluted in 200 μl of blocking solution for a further 3-hour incubation at room temperature with weak stirring.

With both selection methods, the beads were washed 10 times with 1 ml of washing solution (PBS, 1% TritonX100, 10 mM MgSO₄). The bound bacteriophages were amplified for infection of BB4 cells added directly to the beads (1.2 ml per selection) and subsequent 30-minute incubation at room temperature. 12 ml of NZY-Top Agar (Sambrook et al., 1989, Molecular Cloning: a laboratory manual, Cold Spring Harbor Laboratory Press, NY) were added to the mixture of beads and cells and immediately poured onto NZY plates (2 15-cm Petri capsules for selection). The plates were incubated for 12-16 hours at 37° C. The next day the phages were collected from the plates by means of the addition of 15 ml of SM buffer per plate and stirring for 4 hours at room temperature. The phages were purified by precipitation in PEG/NaCl (20% polyethylene glycol, NaCl 1M) and finally resuspended in 5 ml of SM and stored at +4° C.

Selection of the Library with Human Sera

To identify the specific antigens of T. gondii an affinity selection procedure was used consisting of two “panning” cycles with one or more positive sera (that is to say sera deriving from a patient who tested positive for the presence of antibodies directed against the parasite), followed by an immunological screening procedure carried out with the same sera or, alternatively, by analysis of single clones taken at random from the mixture of selected phages. Preferably, the library was selected with 10 positive sera (T1, T2, T3, T4, T5, T6, T7, T8, T9 and T10), generating, after a single selection cycle, the corresponding mixtures p1^(I), p2^(I), p3^(I), p4^(I), p5^(I), p6^(I), p7^(I), p8^(I), p9^(I) and p10^(I). Each mixture was then subjected to a second affinity selection cycle with the same serum, according to the first strategy mentioned above, giving rise to a second series of mixtures (called p1^(II), p2^(II), p3^(II), p4^(II), p5^(II), p6^(II), p7^(II), p8^(II), p9^(II) and p10^(II)). The initial characterisation by means of an enzyme-linked immunosorbent assay (Phage-ELISA) showed that some of the mixtures were more reactive with the corresponding serum used for the selection, thus confirming the efficacy of the library and the affinity selection procedure. Various positive clones were identified by means of immunoplate screening per plaque of reactive mixtures.

Phage-ELISA

Multi-well plates (Immunoplate Maxisorb, Nunc, Denmark) were coated, incubating 100 μl/well of anti-lambda polyclonal antibodies overnight at 4° C. with a concentration of 0.7 μg/ml in NaHCO₃ 50 mM, pH 9.6. After eliminating the coating solution, the plates were incubated with 250 μl of blocking solution (5% skimmed milk powder in PBS, 0.05% Tween-20). The plates were then washed twice with washing buffer (PBS, 0.05% Tween-20). A mixture of 100 μl of blocking solution containing phage lysate (diluted 1:1) was added to each well and incubated for 60 minutes at 37° C. 1 μl of human serum was preincubated for 30 minutes at room temperature with 10⁹ wild-type phage particles, 1 μl of rabbit serum, 1 μl of bacterial extract of BB4 cells, 1 μl of foetal bovine serum in 100 μl of blocking solution. The plates were washed 5 times after incubation with the phage lysate and then incubated with the serum solution for 60 minutes at 37° C. The plates were then washed 5 times and incubated in blocking solution containing human anti-immunoglobulin antibodies conjugated with the enzyme peroxidase (Sigma-Aldrich, USA) diluted 1:10000 and rabbit serum diluted 1:40. After 30 minutes' incubation the plates were washed 5 times and the peroxidase activity was measured with 100 μl of TMB liquid substrate (Sigma-Aldrich, USA). After 15 minutes' development, the reaction was stopped by adding 25 μl of H₂SO₄ 2M. Lastly, the plates were analysed using an automatic ELISA reader (Multiskan, Labsystem, Finland) and the results were expressed as OD=OD_(450 nm)−OD_(620 nm). The ELISA data were assessed as mean values of two independent assays.

Immunoscreening

Phage plaques were transferred from the bacterial medium to nitrocellulose filters (Schleicher & Schuell, Germany) by means of incubation at room temperature for 60 minutes. The filters were blocked for 60 minutes at room temperature in blocking solution (5% skimmed milk powder in PBS, 0.05% Tween-20). 40 μl of human serum were preincubated with 40 μl of bacterial extract of BB4 cells, 10⁹ wild-type lambda phage particles in 4 ml of blocking solution. After eliminating the blocking solution, the filters were incubated with the serum for 3 hours at room temperature under stirring. The filters were then washed 5 times with washing buffer (PBS, 0.05% Tween-20) and then incubated for 60 minutes at room temperature, alternatively with human anti-IgG antibodies conjugated with alkaline phosphatase (Sigma-Aldrich, USA), or with human anti-IgM antibodies conjugated with alkaline phosphatase (Sigma-Aldrich, USA), both diluted 1:7500 in blocking solution. After washing the filters 5 times, 5 ml of development solution (substrates BCIP and NBT, Sigma-Aldrich, USA) were added and the development was interrupted by washing the filters in water.

Preparation of the Lambda Phage from Lysogenic Cells

Phage clones that proved positive at immunoscreening (direct screening) were isolated from the respective phage plaques and then amplified for subsequent characterisation. The bacterial BB4 cells were grown under stirring at 37° C. up to an optical density OD₆₀₀=1.0 in LB culture medium (Sambrook et al., 1989, Molecular Cloning: a laboratory manual, Cold Spring Harbor Laboratory Press, N.Y.) containing 0.2% maltose and 10 mM MgSO₄, recuperated by centrifuging and resuspended in SM buffer at optical density OD₆₀₀=0.2. 100 μl of cells were infected with recombinant bacteriophages recovered from single plaques, incubated for 20 minutes at room temperature, plated on LB medium with ampicillin (100 μg/ml) and then incubated for 18-20 hours at 32° C. A single bacterial colony was then grown in 10 ml of LB/ampicillin overnight at 32° C. under stirring. 500 ml of LB/ampicillin and MgSO₄ 10 mM were added to 5 ml of the overnight culture and incubated at 32° C. up to an optical density OD₆₀₀=0.6 under vigorous stirring. The culture was then incubated for 15 minutes in a water bath at 45° C. and then at 37° C. for a further 3 hours. After this, the bacteriophages were purified of the bacterial culture according to standard procedures (Sam brook et al., 1989, Molecular Cloning: a laboratory manual, Cold Spring Harbor Laboratory Press, NY) and stored at +4° C.

Lastly, the phage clones were analysed by means of phage-ELISA with a substantial panel of positive and negative sera. Clones whose ELISA value exceeded the background value, as obtained from the sum of the mean of the measurements of the negative sera and three times the standard deviation, were judged to be positive.

The following table 1 gives, by way of examples, the reactivity of a number of the recombinant bacteriophages selected. TABLE 1 Reactivity of phage Reactivity of phage clone clone with positive sera with negative sera Name of clone (positive/total pos.) (negative/total neg.) Tx-4.11 13/21 0/10 TxM-17.2 4/8 1/8  Tx-15.11 11/21 0/10 Tx-1.11 19/21 0/10 Tx-8.0  6/21 0/10 Tx-1.16 20/21 0/10 Tx-9.18  9/21 0/8  Tx-7.11 21/21 0/10 Characterisation of Positive Clones

The clones which showed multiple reactivity with the Toxoplasma gondii positive sera and which presented no reactivity to the negative sera were subsequently sequenced and compared with various databases of sequences currently available (Non-Redundant Genbank CDS, Non-Redundant Database of Genbank Est Division, Non-Redundant Genbank+EMBL+DDBJ+PDB Sequences).

The sequences obtained can be classified in four groups:

-   -   sequences that code for known T. gondii antigen fragments;     -   sequences that code for known proteins which, however, are not         known to be involved in the human antibody response;     -   sequences that code for unknown proteins (e.g. EST);     -   new sequences, not yet figuring in the databases.

The following table 2 gives, by way of examples, the sequences of some of the clones selected: TABLE 2 Name Identi- Classi- of clone Sequence fication fication Tx-4.11 AGTGGAGGGACAGGGCAGGGATTA GRA 1 Dense (SEQ ID GGAATCGGAGAATCTGTAGATTTG known granule No 17) GAGATGATGGGGAACACGTATCGT T. gondii protein GTGGAGAGACCCACAGGCAACCCG antigen GACTTGCTCAAGATCGCCATTAAA GCTTCAGATGGATCGTACAGCGAA GTCGGCAATGTTAACGTGGAGGAG GTGATTGATACTATGAAAAGCATG CAGAGGGACGAGGACATTTTCCTT CGTGCGTTGAACAAAGGCGAAACA GTAGAGGAAGCGATCGAAGACGTG GCTCAAGCAGAAGGGCTTAATTCG GAGCAAACCCTGCAACTGGAAGAT GCAGTGAGCGCGGTGGCGTCTGTT GTTCAAGACGAG TxM-17.2 TACTCTTCACCACGAATAGTTGTT GRA2 Dense (SEQ ID TTGATTAGATATTGCTTCTTCTCC known granule No 18) ACATATCGCCTCACAATGTTCGCC T. gondii protein GTAAAACATTGTTTGCTGGTTGTT antigen GCCGTTGGCGCCCTGGTCAACGTC TCGGTGAGGGCTGCCGAGTTTTCC GGAGTTGTTAACCAGGGACCT Tx-15.11 GCTGCCTTGGGAGGCCTTGCGGCG GRA 3 Dense (SEQ ID GATCAGCCTGAAAATCATCAGGCT granule No 19) CTTGCAGAACCAGTTACGGGTGTG protein— GGGGAAGCAGGAGTGTCCCCCGTC unknown AACGAAGCTGGTGAGTCATACAGT as anti- TCTGCAACTTCGGGTGTCCAAGAA gen in GCTACCGCCCCAGGTGCAGTGCTC human CTGGACGCAATCGATGCCGAGTCG response GATAAGGTGGACAATCAGGCGGAG GGAGGTGAGCGTATGAAGAAGGTC GAAGAGGAGTTGTCGTTATTGAGG CGGGAATTATATGATCGCACAGAT CGCCCTGGT Tx-1.11 CAGTTCGCTACCGCGGCCACCGCG GRA 7 Dense (SEQ ID TCAGATGACGAACTGATGAGTCGA known granule No 20) ATCCGAAATTCTGACTTTTTCGAT T. gondii protein GGTCAAGCACCCGTTGACAGTCTC antigen AGACCGACGAACGCCGGTGTCGAC TCGAAAGGGACCGACGATCACCTC ACCACCAGCATGGATAAGGCATCT GTAGAGAGTCAGCTTCCGAGAAGA GAGCCATTGGAGACGGAGCCAGAT GAACAAGAAGAAGTTCAT Tx-8.0 GAGAACCCGGTGAGACCGCCTCCT GRA8 Dense (SEQ ID CCCGGTTTCCATCCAAGCGTTATT known granule No 21) CCCAATCCCCCGTACCCGCTGGGC T. gondii protein ACTCCAGCGGGCATGCCACAGCCA antigen GAGGTTCC Tx-1.16 AGGAGGACTGGATGTCATGCCTTC MIC 3 Micro- (SEQ ID AGGGAGAACTGCAGCCCTGGTAGA neme No 22) TGTATTGATGACGCCTCGCATGAG protein— AATGGCTACACCTGCGAGTGCCCC unknown ACAGGGTACTCACGTGAGGTGACT as anti- TCCAAGGCGGAGGAGTCGTGTGTG gen in GAAGGAGTCGAAGTCACGCTGGCT human GAGAAATGCGAGAAGGAATTCGGC response ATCAGCGCGTCATCCTGCAAATGC GAT Tx-9.18 GCACCCACTCAATCTGAAATGAAA MIC 5 Micro- (SEQ ID GAATTCCAAGAGGAAATCAAAGAA known neme No 23) GGGGTGGAGGAAACAAAGCATGAA T. gondii protein GACGATCCTGAGATGACGCGGCTC antigen ATGGTGACCGAGAAGCAGGAGAGC AAAAATTTCAGCAAGATGGCGAAA TCCCAGAGTTTTAGCACGCGAATC GAAGAGCTCGGGGGATCCATTTCG TTTCTAACTGAAACGGGGGTCACA ATGATCGAGTTGCCCAAAACTGTC AGTGAACATGACATGGACCAACTA CTCCAC Tx-7.11 GTTATGGCATCGGATCCCCCTCTT SAG 1 Surface (SEQ ID GTTGCCAATCAAGTTGTCACCTGC known protein No 24) CCAGATAAAAAATCGACAGCCGCG T. gondii GTCATTCTCACACCGACGGAGAAC antigen CACTTCACTCTCAAGTGCCCTAAA ACAGCGCTCACAGAGCCTCCCACT CTTGCGTACTCACCCAACAGGCAA ATCTGCCCAGCGGGTACTACAAGT AGCTGTACATCAAAGCTGTAACAT TGAGCTCCTTGATTCCTGAAGCAG AAGATAGCTGGTGGACGGGGGATT CTGCTAGTCTCGACACGGCAGGCA TCAAACTCACAGTTCCAATCGAGA AGTTCCCCGTGACAACGCAGACGT TTGTGGTCGGTTGCATCAAGGGAG ACGACGCACAGAGTTGTATGGTCA CGGTGACAGTACAAGCCAGAGCCT CATCGGTCGTCAATAATGTCGCAA GGTGCTCCTATGGTGCGGACAGC Tx-4.18 CCATCGGTCGTCAATAATGTCGCA SAG 1 Surface (SEQ ID AGGTGCTCCTACGGTGCAGACAGC known protein No 25) ACTCTTGGTCCTGTCAAGTTGTCT T. gondii GCGGAAGGACCCACTACAATGACC antigen CTCGTGTGCGGGAAAGATGGAGTC AAAGTTCCTCAAGACAACAATCAG TACTGTTCCGGGACGACGCTGACT GGTTGCAACGAGAAATCGTTCAAA GATATTTTGCCAAAATTAACTGAG AACCCGTGGCAGGGTAACGCTTCG AGTGATAAGGGTGCCACGCTAACG ATCAAGAAGGAAGCATTTCCAGCC GAGTCAAAAAGCGTCATTATTGGA TGCACAGGGGGATCGCCTGAGAAG CATCACTGTACCGTGAAACTGGAG TTTGCCGGGGCTGCAGGGTCAGCA AAATCGGCT

The clone Tx-4.11 constitutes a fragment of the antigen GRA1 (Cesbron-Delauw et al., 1989, Proc. Natl. Acad. Sci. USA 86:7537-7541) but has never been identified as an “antigen fragment” of the protein in the human humoral response. Said clone has the amino acid sequence (SEQ ID No 26) SGGTGQGLGIGESVDLEMMGNTYRVERPTGNPDLLKIMKASDGSYSEVGNVNVEEVIDTMKSMQRDEDIFLR ALNKGETVEEAIEDVAQAEGLNSEQTLQLEDAVSAVASVVQDE and its use as a fragment containing an epitope is covered by the present invention.

The clone TxM-17.2 constitutes a fragment of the antigen GRA2 (Prince et al., 1989, Mol. Biochem. Parasitol., 34: 3-14) but has never been identified as an “antigen fragment” of the protein in the human humoral response. Said clone has the amino acid sequence (SEQ ID No 27) YSSPRIVVLIRYCFFSTYRLTMFAVKHCLLVVAVGALVNVSVRAAEFSGVVNQGP and its use a fragment containing an epitope is covered by the present invention.

The clone Tx-15.11 constitutes a fragment of the gene GRA3 (Bermudes et al., 1994, Mol. Biochem. Parasitol., 68: 247-257) and has never been identified as an antigen in the human antibody response. Said clone has the amino acid sequence (SEQ ID No 28) AALGGLAADQPENHQALAEPVTGVGEAGVSPVNEAGESYSSATSG VQEATAPGAVLLDAIDAESDKVDNQAEGGERMKKVEEELSLLRRE LYDRTDRPG and its use as a fragment containing an epitope is covered by the present invention.

The clone Tx-1.11 constitutes a fragment of the antigen GRA7 (Bonhomme et al., 1998, J. Histochem. Cytochem. 46, 1411-1421) and has never been identified as an “antigen fragment” of the protein in the human humoral response. Said clone has the amino acid sequence (SEQ ID No 29) FATAATASDDELMSRIRNSDFFDGQAPVDSLRPTNAGVDSKGTDDHLTTSMDKASVESQLPRREPLETEPDEQEEVHF and its use as a fragment containing an epitope is covered by the present invention.

The clone Tx-8.0 constitutes a fragment of the antigen GRA8 (Kimberly et al., 2000, Mol. Biochem. Parasitol., 105: 25-37) and has never been identified as an “antigen fragment” of the protein in the human humoral response. Said clone has the amino acid sequence (SEQ ID No 30) ENPVRPPPPGFHPSVIPNPPYPLGTPAGMPQPEVP and its use as a fragment containing an epitope is covered by the present invention.

The clone Tx-1.16 constitutes a fragment of the gene MIC3 (Garcia-Réguet et al., 2000, Cellular Microbiol., 2: 353-364) and has never been identified as an antigen in the human antibody response. Said clone has the amino acid sequence (SEQ ID No 31) RRTGCHAFRENCSPGRCIDDASHENGYTCECPTGYSREVTSKAEESCVEGVEVTLAEKCEKE FGISASSCKCD and its use as a fragment containing an epitope is covered by the present invention.

The clone Tx-9.18 constitutes a fragment of the antigen MIC5 (Brydges et al., 2000, Mol. Biochem. Parasitol., 111: 51-66) but has never been identified as an “antigen fragment” of the protein in the human humoral response. Said clone has the amino acid sequence (SEQ ID No 32) APTQSEMKEFQEEIKEGVEETKHEDDPEMTRLMVTEKQESKNFSKMAKSQSFSTRIEELGGSISFLTETGVTMIELPKTVSEHDMDQLL H and its use as a fragment containing an epitope is covered by the present invention.

The clone Tx-7.11 constitutes a fragment of the antigen SAG1 (Burg et al., 1988, J. Immunol., 141:3584-3591) but has never been identified as an “antigen fragment” of the protein in the human humoral response. Said clone has the amino acid sequence (SEQ ID No 33) VMASDPPLVANQVVTCPDKKSTAAVILTPTENHFTLKCPKTALTEPPTLAYSPNR QICPAGTTSSCTSKAVTLSSLIPEAEDSWWTGDSASLDTAGIKLTVPI EKFPVTTQTFVVGCIKGDDAQSCMVTVTVQARASSVVNNVARCSYG ADS and its use a fragment containing an epitope is covered by the present invention.

The clone Tx-4.18 constitutes a fragment of the antigen SAG1 (Burg et al., 1988, J. Immunol., 141:3584-3591) but has never been identified as an “antigen fragment” of the protein in the human humoral response. Said clone has the amino acid sequence (SEQ ID No 34) PSVVNNVARCSYGADSTLGPVKLSAEGPTTMTLVCGKDGVKVPQDNNQYCSGTT LTGCNEKSFKDILPKLTENPWQGNASSDKGATLTIKKEAFPAESKS VIIGCTGGSPEKHHCTVKLEFAGAAGSAKSA and its use as a fragment containing an epitope is covered by the present invention.

Expression of cDNA Fragments Selected from the Library as Fusion Products with GST

The plasmid pGEX-SN was constructed by cloning the DNA fragment deriving from the hybridisation of the synthetic oligo-nucleotides K108 5′-GATCCTTACTAGTTTTAGTAGCGGCCGCGGG-3′ (SEQ ID No 35) and K109 5′-AATTCCCGCGGCCGCTACTAAAACTAGTAAG-3′ (SEQ ID No 36) in the BamHI and EcoRI sites of plasmid pGEX-3X (Smith and Johnson, 1988, Gene, 67, 31-40).

The phage clones for which specific reactivity with sera of patients testing positive for Toxoplasma gondii was demonstrated, were amplified and then analysed with a substantial panel of positive and negative sera. After this ELISA study, DNA inserts of clones that showed multiple reactivity with Toxoplasma gondii-positive sera and presented no reactivity with the negative sera were cloned as fusion products with the protein Glutathione Sulphur Transferase (GST) and expressed in the cytoplasma of bacterial cells, for the purposes of determining their specificity and selectivity. To produce the fusion proteins each clone was amplified from a single phage plaque by PCR, using the following oligonucleotides: K47 5′-GGGCACTCGACCGGAATTATCG-3′ (SEQ ID No 37) and K85 5′-GGGTAAAGGTTTCTTTGCTCG-3′ (SEQ ID No 38). The resulting fragment was then purified by means of the “Qiagen Purification Kit” (Qiagen, CA, USA), digested with the restriction enzymes SpeI and NotI and cloned in the vector pGEX-SN to generate the fusion with GST. The corresponding recombinant proteins were then expressed in E. coli and purified by affinity using Glutathione-Sepharose resin (Amersham Pharmacia Biotech, Sweden) and following standard protocols (Sambrook et al., 1989, Molecular Cloning, Cold Spring Harbor Laboratory Press, Cold Spring Harbor).

The following table 3, by way of examples, presents the reactivity with negative and positive sera of a number of the clones selected, assayed in the form of fusion proteins: TABLE 3 Reactivity of GST Reactivity of GST fusion protein fusion protein with positive sera with negative sera Name of clone (pos./total neg.) (neg./total neg.) Tx-4.11 32/40 1/28 Tx-15.11 22/40 0/28 Tx-1.11 27/40 0/28 Tx-8.0 13/40 0/28 Tx-1.16 31/40 0/28 Tx-9.18  3/40 1/28 Tx-7.11 39/40 3/28 Tx-4.18 21/40 1/28 IgG Avidity for Determining the Time of Infection

Measurement of the binding force between immunoglobulin G (IgG) and the T. gondii-specific antigens (IgG avidity) is a diagnostic method used to establish the time of infection: IgG avidity is lower during the acute phase of the infection and then tends to increase in the course of time (Hedman et al. 1989 J. Infect. Dis. 159, 736-740). Evaluation of IgG avidity is based on an enzyme-linked immunosorbent assay (ELISA) in which the immunoglobulins are “detached” from the antigen by washing with a urea denaturing solution. A mathematical calculation based on the reactivity before and after the denaturing washing makes it possible to estimate serum IgG avidity (Jenum et al., 1997. J. Clin. Microbiol. 35, 1972-1977). To evaluate the antigenic properties of the protein fragments described in the present invention, an IgG avidity test based on recombinant antigen fragments was developed and the results obtained were compared with the assay performed with a commercial kit (Toxo-IgG avidity kit, bioMerièux, France) that employs the whole parasite extract as antigen.

For the avidity analysis 27 sera coming from women who contracted primary Toxoplasmosis during pregnancy and collected at different times after infection were used. The infection was diagnosed by seroconversion during gestation, taking into consideration last negative and first positive samples. For each sample the specific IgG and IgM levels for T. gondii and the IgG avidity were determined by means of the use of commercial kits (LDBIO Diagnostic, France; bioMerièux, France). Multi-well plates (Nunc, Denmark) were incubated overnight at 4° C. with a solution of NaHCO₃ 50 mM, pH 9.6 containing the antigen fragments expressed as GST fusion proteins at a final concentration of 5 μg/ml. The plates were blocked with 200 μl of blocking solution (5% skimmed milk powder in PBS, 0.05% Tween-20) and then washed 5 times with washing buffer (PBS, 0.05% tween-20). Serial dilutions of serum (1:50, 1:200, 1:800, 1:3200) in 100 μl of blocking solution were incubated on plates for 60 minutes at 37° C. The plates were then incubated with a denaturing solution of urea 6M in PBS/0.02% Tween-20 for 30 minutes at 37° C. In parallel, for every sample, the same dilutions of serum were effected and the wells concerned were incubated with normal washing buffer. 100 μl of blocking solution containing human anti-IgG antibodies conjugated with the enzyme alkaline phosphatase (diluted 1:10000) were added to the plates. After 30 minutes' incubation at 37° C. the plates were washed and the enzyme activity was determined with 100 ill of development solution (10% diethanolamine pH 9.8, 0.5 mM MgCl₂, 0.05% NaN₃) containing the reaction substrate p-nitrophenylphosphate (Sigma-Aldrich, USA). The enzyme activity was measured at optical densities of 405 nm and 620 nm by means of an automatic ELISA OD reader (Multiskan Labsystem, Finland) and the avidity calculation was done according to the mathematical analysis described in the literature (Jenum et al., 1997. J. Clin. Microbiol. 35, 1972-1977).

The following table 4 gives, by way of examples, the avidity of the human sera for a number of the antigen fragments selected.

The values should be interpreted as follows (commercial kit criterion):

<15% Low avidity: acute infection in the last three months

15%-30% Borderline avidity: probable primary infection within 3-6 months is possible

>30% High avidity: excludes primary infection within the last three months

EXAMPLE 2

Using the vector λKM4 of Example 1, a library of DNA fragments of known Toxoplasma gondii genes was constructed.

Cells of Toxoplasma gondii (10⁶ parasites, strain ME49) were grown in vitro in monkey kidney cells (“VERO” African green monkey cells) using DMEM culture medium containing 10% foetal bovine serum, 2 mM glutamine and 0.05 mg/ml gentamicin (Gibco BRL, Canada). To have both forms of the parasite (tachyzoites and bradyzoites) present in the cell cultures, an experimental protocol was used based on the change in pH of the culture medium (Soete et al., 1994, Experimental Parasitology, 78, 361-370). The parasites were collected after complete lysis of the host cells and purified by filtration (filter porosity 3 μm) followed by centrifuging. 2 μg of mRNA were isolated from 5×10⁶ parasites using the “QuickPrep Micro mRNA Purification Kit” (Amersham Pharmacia Biotech, Sweden) and following the manufacturer's instructions. cDNA was synthesised from 200 ng of poly(A)+ RNA using the “SMART cDNA Library Construction Kit” (Clontech, CA, USA) and following the manufacturer's instructions. Genomic DNA was purified from the remaining 5×10⁶ cells using standard procedures (Sambrook et al., 1989, Molecular Cloning: a laboratory manual, Cold Spring Harbor Laboratory Press, NY) and stored at −20° C.

For the construction of the expression/exposure library the following genes, expressed only in the bradyzoite stage, were amplified by means of PCR with specific oligonucleotides:

-   1—SAG2D (Lekutis et al., 2000, Experimental Parasitology, 96, 89-96)     was obtained from genomic DNA using the oligonucleotides     5′-ATGGCGGCTGCACACTCG-3′ (SEQ ID No 39) and     5′-GAACATATTCCCTGTCACCAATG-3′ (SEQ ID No 40); -   2—SAG4 (Odberg-Ferragut et al., 1996, Molecular and Biochemical     Parasitology, 82, 237-244) was obtained from genomic DNA using the     oligonucleotides 5′-ATGACGAAAAATAAAATTCTTCTC-3′ (SEQ ID No 41) and     5′-CATTGATATCAACACAAAGGCC-3′ (SEQ ID No 42) -   3—BSR4 (Manger et al., 1998, Infection and Immunity, 66, 2237-2244)     was obtained from genomic DNA using the oligonucleotides     5′-ATGGTGATGATGGGCAGCATG-3′ (SEQ ID No 43) and     5′-CGGCGGCCGCGCTAGAGG-3′ (SEQ ID No 44); -   4—MAG1 (Parmley et al., 1994, Molecular and Biochemical     Parasitology, 66, 283-296) was obtained from genomic DNA using the     oligonucleotides 5′-CGTTGGATCCTTGGATTGAGCCAAAGGGTGCCAG-3′ (SEQ ID     No 45) and 5′-CCCAGAATTCTCAAGCTGCCTGTTCCGCTAAGATCTG-3′ (SEQ ID No     46); -   5—LDH2 (Yang and Parmley, 1997, Gene, 184, 1-12) was obtained from     cDNA using the oligonucleotides 5′-ATGACGGGTACCGTTAGCA-G-3′ (SEQ ID     No 47) and 5′-ACCCAGCGCCGCTAAACTC-3′ (SEQ ID No 48); -   6—ENO1 (Dzierszinski et al., 2001, Journal of Molecular Biology,     309, 1017-1027) was obtained from genomic DNA using the     oligonucleotides 5′-ATGGTGGTTATCAAGGACATCG-3′ (SEQ ID No 49) and     5′-TTTTGGGTGTCGAAAGCTCTC-3′ (SEQ ID No 50); -   7—BAG1 (Bohne et al., 1995, Molecular Microbiology, 16, 1221-1230)     was obtained from cDNA using the oligonucleotides     5′-ATGGCGCCGTCAGCATCG-3′ (SEQ ID No 51) and     5′-CTTCACGCTGATTTGTTGCTTTG-3′ (SEQ ID No 52); -   8—p-ATPase (Holpert et al., 2001, Molecular and Biochemical     Parasitology, 112, 293-296) was obtained from genomic DNA using the     oligonucleotides 5′-ATGGACGAAGCGAGCAGAAGG-3′ (SEQ ID No 53) and     5′-ACGCGTGATCGAAGGAACCG-3′ (SEQ ID No 54).

10 μg of DNA deriving from a mixture of the amplification products of the above-mentioned genes were fragmented randomly using 0.5 ng of the endonuclease DNaseI (Sigma-Aldrich, USA). The mixture of DNA and DNaseI was incubated for 20 minutes at 15° C. and the DNA fragments were purified by means of the “QIAquick PCR Purification Kit” (Qiagen, CA, USA), following the manufacturer's instructions. The 3 μg ends of the cDNA fragments were “flattened” by incubating the DNA with 9 units of the enzyme T4 DNA polymerase (New England Biolabs, MA, USA) for 60 minutes at 15° C. The fragments were then purified by means of extraction in phenol/chloroform and subsequent precipitation in ethanol. 500 ng of the resulting DNA were bound with a 20-fold molar excess of “synthetic adaptors” for the purposes of adding the restriction sites SpeI and NotI to the ends of the fragments. Six adaptors were used, obtained by hybridisation of the following pairs of oligonucleotides: K185 5′-CTAGTCGTGCTGGCCAGC-3′ (SEQ ID No 5) and K186 5′-GCTGGCCAGCACGA-3′ (SEQ ID No 6); K187 5′-CTAGTCGTGCTGGCCA GCT-3′ (SEQ ID No 7) and K188 5′-AGCTGGCCAGCACGA-3′ (SEQ ID No 8); K189 5′-CTAGTCGT GCTGGCCAGCTG-3′(SEQ ID No 9) and K190 5′-CAGCTGGCCAGCACGA-3′ (SEQ ID No 10); K191 5′-TCTGGTGGCGGTAGC-3′ (SEQ ID No 11) and K192 5′-GGCCGCTACCGCCACCAGA-3′(SEQ ID No 12); K193 5′-TTCTGGTGGCGGTAGC-3′ (SEQ ID No 13) and K194 5′-GGCCGCTACCGCCACCAGAA-3′ (SEQ ID No 14); K195 5′-TTTCTGGTGGCGGTAGC-3′ SEQ ID No 15) and K196 5′-GGCCGCTACCGCCACCAGAAA-3′ (SEQ ID No 16). The excess of unligated adaptors was removed from the ligation mixture by electropheresis on 2% agarose gel and the cDNA fragments with molecular weights ranging from 250 bp to 1000 bp were excised from the gel and purified by means of the “Qiaquick gel extraction kit” (Qiagen, CA, USA) following the manufacturer's instructions. The vector λKM4 was digested with SpeI/NotI and for the construction of the library 6 ligation mixtures were performed, each containing 0.4 μg of vector and approximately 7 ng of insert. After overnight incubation at 4° C. the ligation mixtures were packaged in vitro with the “Ready-To-Go lambda packaging kit” (Amersham Pharmacia Biotech, Sweden) and plated for infection of BB4 cells (bacterial cells of E. coli strain BB4; Sambrook et al., 1989, Molecular Cloning: a laboratory manual, Cold Spring Harbor Laboratory Press, NY). After overnight incubation at 37° C. the phage was eluted from the plates with SM buffer (Sambrook et al., 1989, Molecular Cloning: a laboratory manual, Cold Spring Harbor Laboratory Press, NY), purified, concentrated and stored at −80° C. in SM buffer containing 7% dimethylsulphoxide. The complexity of the library calculated as the number of total independent clones with inserts was 10⁶ clones.

Affinity selection, phage-ELISA, immunoscreening and phage clones preparation were performed exactly as described in Example 1.

The following table 5 gives, by way of examples, the reactivity of a number of the recombinant bacteriophages selected. TABLE 5 Reactivity of phage Reactivity of phage clone with positive clone with negative sera (positive/ sera (negative/ Name of clone total positive) total negative) TxB-cl21.2 10/20 1/10 TxB-cl26.3 12/20 0/10 TxB-44.3 10/20 0/10 TxB-7.1  8/20 0/10 TxB-9.1  1/20 1/10 TxB-12.1  5/20 0/10 Characterisation of Positive Clones

The clones which showed multiple reactivity with the Toxoplasma gondii positive sera and which presented no reactivity with the negative sera were subsequently sequenced and compared with the sequences of the genes used to construct the library.

The following table 6 gives, by way of examples, the sequences of some of the clones selected: TABLE 6 Name of clone Sequence TxB-26.3 GGATTGAGCCAAAGGGTGCCAGAGCTACCAGAAGT (SEQ ID No 55) GGAGCCCTTTGATGAAGTAGGCACGGGAGCTCGAC GGTCCGGGTCCATTGCGACCCTTCTTCCACAAGAC GCTGTTTTATATGAGAACTCAGAGGACGTTGCCGT TCCGAGTGATTCAGCATCGACCCCGTCATACTTTC ATGTGGAATCTCCAAGTGCTAGTGTGGAAGCCGCG ACTGCCGCTGTGGGAGAGGTGGTGCCGGACTGTGA AGAACAACAGGAACAGGGTGACACGACGTTATCCG ATCACGATTTCCATTCA TxB-cl7.1 TCTTCAGAAAGATGACGTAACCATAGAAGTCGACA (SEQ ID No 56) ACGGAGCCATCGTTATCAAAGGAGAGAAGACCTCG AAAGAAGCGGAGAAAGTGGACGATGGCAAAACAAA GAACATTTTGACTGAGCGAGTGTCCGGTTATTTTG CGCGCCGGTTCCAGCTCCCGAGTAATTACAAGCCC GACGGAATCAGTGCGGCAATGGACAACGGCGTTCT ACGTGTCACGATCAAGGTCGAGGATTCAGGGGGCG CAAAGCAACAAATCAGCGTG

The clone TxB-cl26.3 constitutes a fragment of the gene MAG1, a 65 kDa protein of the matrix and wall of T. gondii cysts (Parmley et al., 1994, Molecular and Biochemical Parasitology, 66, 283-296), the protein product of which has never been identified as an “antigen fragment” in the human humoral response. Said clone has the amino acid sequence GLSQRVPELPEVEPFDEVGTGARRSGSIATLLPQDAVLYENSEDVAVPSDSASTPSYFHVESPSASVEAATGAVGEVVPDCEE QQEQGDTTLSDHDFH (SEQ ID No 57) and its use as a fragment containing an epitope is covered by the present invention.

The clone TxB-cl7.1 constitutes a fragment of the gene BAG1, a 30 kDa protein of the heat shock protein family of T. gondii (Bohne et al., 1995, Molecular Microbiology, 16, 1221-1230), the protein product of which has never been identified as an “antigen fragment” in the human humoral response. Said clone has the amino acid sequence LNPIDDMLFETALTANEMMEDITWRPRVDVEFDSKKKEMIILADLP GLQKDDVTIEVDNGAIVIKGEKTSKEAEKVDDGKTKNILTERVSGY FARRFQLPSNYKPDGISAAMDNGVLRVTIKVEDSGGAKQQISV (SEQ ID No 58) and its use as a fragment containing an epitope is covered by the present invention.

Expression of DNA Fragments Selected from the Library as Fusion Products with GST

The phage clones for which specific reactivity with sera of patients testing positive for Toxoplasma gondii was demonstrated, were amplified and then analysed with a substantial panel of positive and negative sera. After this ELISA study, the clones that showed multiple reactivity with Toxoplasma gondii-positive sera and presented no reactivity with the negative sera were cloned as fusion products with the protein Glutathione Sulphur Transferase (GST) and expressed in bacterial cells, for the purposes of determining their specificity and selectivity. To produce the fusion proteins each clone was amplified from a single phage plaque by PCR, using the following oligonucleotides: K47 5′-GGGCACTCGACCGGAATTATCG-3′ (SEQ ID No 37) and K85 5′-GGGTAAAGGTTTCTTTGCTCG-3′ (SEQ ID No 38). The resulting fragment was then purified by means of the “Qiagen Purification Kit” (Qiagen, CA, USA), digested with the restriction enzymes SpeI and NotI and cloned in the vector PGEX-SN to generate the fusion with GST. The corresponding recombinant proteins were then expressed in E. coli and purified by affinity using Glutathione-Sepharose resin (Amersham Pharmacia Biotech, Sweden) and following standard protocols (Sambrook et al., 1989, Molecular Cloning, Cold Spring Harbor Laboratory Press, Cold Spring Harbor).

The following table 7, by way of examples, presents the reactivity with negative and positive sera of a number of the clones selected, assayed in the form of fusion proteins: TABLE 7 Reactivity of GST Reactivity of GST fusion protein with fusion portein with positive sera negative sera Name of clone (pos./total neg.) (neg./totale neg.) TxB-cl26.3 30/34 0/32 TxB-cl7.1 17/34 0/32

EXAMPLE 3

By using the same strategy described in Example 2, a gene collection of DNA encoding for protein products of the Toxoplasma gondii microneme family was used to construct a “microneme-display library”.

For the construction of the microneme-library the following genes were amplified by means of PCR with specific oligonucleotides:

-   1—MIC2 (Wan et al, 1997, Mol. Biochem. Parasitol. 84: 203-214) was     obtained from single strand cDNA using the oligonucleotides     5′-ATGAGACTCCAACCGAGGCC-3′ (SEQ ID No 69) and     5′-CTGCCTGACTCTTTCTTGGACTG-3′ (SEQ ID No 70); -   2—M2AP (Rabenau et al., 2001, Mol. Microbiol. 41: 537-547) was     obtained from single strand cDNA using the oligonucleotides     5′-GGAAAGTTGGAAATCCGGCGGC-3′ (SEQ ID No 71) and     5′-CGCCTCATCGTCACTCGGC-3′ (SEQ ID No 72) -   3—MIC4 (Brecht et al., 2001, J. Biol. Chem. 276:4119-412) was     obtained from single strand cDNA using the oligonucleotides     5′-ATGAGAGCGTCGCTCCCGG-3′ (SEQ ID No 73) and     5′-GTGTCTTTCGCTTCAAGCACCTG-3′ (SEQ ID No 74); -   4—AMA1 (Hehl et al., 2000, Infect. Immun. 68:7078-7086) was obtained     from single strand cDNA using the oligonucleotides     5′-ATGGGGCTCGTGGGCGTAC-3′ (SEQ ID No 75) and     5′-GATCAACGCAGTGTTAGAGCCAC-3′ (SEQ ID No 76);

10 μg of DNA deriving from a mixture of the amplification products of the above-mentioned genes were fragmented randomly using 0.5 ng of the endonuclease DNaseI (Sigma-Aldrich, USA). The mixture of DNA and DNaseI was incubated for 20 minutes at 15° C. and the DNA fragments were purified by means of the “QIAquick PCR Purification Kit” (Qiagen, CA, USA), following the manufacturer's instructions. Consequent steps for the construction of the microneme-library, and for the affinity selection were performed by following the procedure described in Example 2.

Selection of the Microneme-Library with Sera of Infants Who Were Infected by T. gondii During Pregnancy

To identify the antigenic domains of the T. gondii microneme proteins an affinity selection procedure was used consisting of two “panning” cycles with four sera collected from infants who were congenitally infected by the parasite, followed by an immunological screening procedure carried out with the same sera. The library was selected with sera T1, T2, T3, T4, generating, after a single selection cycle, the corresponding mixtures p1^(I), p2^(I), p3^(I) and p4^(I). Each mixture was then subjected to a second affinity selection cycle with the same serum, giving rise to a second series of mixtures (called p1^(II)p2^(II), p3^(II) and p4^(II)). Various positive clones were identified by means of immunoplate screening per plaque of reactive mixtures.

Phage-Elisa, immunoscreening, and the preparation of phage clones were subsequently performed exactly as described in Examples 1 and 2.

The following table 8 gives, by way of examples, the reactivity of a number of the recombinant bacteriophages selected. TABLE 8 Reactivity of phage Reactivity of phage clone with positive clone with negative sera (positive/ sera (negative/ Name of clone total positive) total negative) Tx-2.a 13/16 0/10 Tx-1.b 11/16 0/10 Tx-11.b 12/16 0/10 Tx-13.b  9/16 0/10 Tx-15.b  9/16 0/10 Characterisation of Positive Clones

The following table 9 gives the sequences of the clones selected: TABLE 9 Name of the Identi- Classi- clone Sequence fication fication Tx-2.a CCCCAGGATGCCATTTGCTCGGATT MIC2 Micro- (SEQ ID GGTCCGCATGGAGCCCCTGCAGTGT neme No 59) ATCCTGCGGTGACGGAAGCCAAATC protein AGGACGCGAACTGAGGTTTCTGCTC unknown CGCAACCTGGAACACCAACATGTCC as anti- GGACTGCCCTGCGCCCATGGGAAGG gen in ACTTGCGTGGAACAAGGCGGACTTG human AAGAAATCCGTGAATGCAGTGCGGG response GGTATGTGCTGTTGACGCTGGATGT GGCGTCTGGGTT Tx-1.b CCGTGTCCAATTAATGCAACTTGCG MIC2 Micro- (SEQ ID GTCAGTTTGAAGAATGGAGTACATG neme No 60) CTCGGTCTCATGTGGTGGTGGACTG protein AAAACGAGGTCGAGGAACCCTTGGA unknown ATGAAGACCAACAACATGGAGGACT as anti- ATCCTGCGAGCAGCAGCATCCTGGT gen in GGGCGGACGGAAACGGTAACTTGCA human ATCCTCAAGCGTGTCCTGTGGATGA response ACGACCGGGGGAGTGGGCAGAGTGG GGGGAATGTAGTGTCACGTGCGGCG ACGGAGTGCGAGAGCGCAGGCGCGG GAAAAGTCTAGTTGAGGCTAAATTC GGCGGACGCACCATTGATCAGCAGA ATGAGGCTCTTCCGGAAGACTTAAA AATCAAAAACGTCGAGTATGAGCCA TGTTCGTATCCTGCTTGTGGAGCTT CCTGCACGTACGTCTGGAGTGACTG GAACAAG Tx-11.b AACGAACCGGTGGCCCTAGCTCAGC M2AP Micro- (SEQ ID TCAGCACATTCCTCGAGCTCGTCGA neme No 61) GGTGCCATGTAACTCTGTTCATGTT protein CAGGGGGTGATGACCCCGAATCAAA unknown TGGTCAAAGTGACTGGTGCAGGATG as anti- GGATAATGGCGTTCTCGAGTTCTAT gen in GTCACGAGGCCAACGAAGACAGGCG human GGGACACAAGCCGAAGCCATCTTGC response GTCGATCATGTGTTATTCCAAGGAC ATTGACGGCGTGCCGTCAGACAAAG CGGGAAAGTGCTTTCTGAAGAACTT TTCTGGTGAAGACTCGTCGGAAATA GACGAAAAAGAAGTATCTCTACCCA TCAAGAGCCACAACGATGCGTTCAT GTTCGTTTGTTCTTCAAATGATGGA TCCGCACTCCAGTGTGATGTTTTCG CCCTTGATAACACCAACTCTAGCGA CGGGTGGAAAGTGAATACCGTGGAT CTTGGCGTCAGCGTTAGTCCGGATT TGGCATTCGGACTCACTGCAGATGG GGTCAAGGTGAAGAAGTTGTACGCA AGCAGCGGCCTGACAGCGATCAACG ACGACCCTTCCTTGGGGTGCAAGGC TCCTCCCCATTCTCCGCCGGCCGGA GAGGAACCGAGTTTGCCGTCGCCTG AAAACAGCGGGTCTGCAACACCAGC GGAAGAAAGTCCGTCTGAGTCTGAA TCT Tx-13.b CTTCGCGGGTACAGGTTCGGTGTTT AMA1 Micro- (SEQ ID GGAAGAAAGGCCGTTGCCTCGACTA neme No 62) CACTGAATTGACCGACACTGTGATA protein GAACGTGTTGAGTCAAAGGCACAGT unknown GCTGGGTGAAAACCTTTGAAAACGA as anti- CGGGGTCGCGAGTGACCAACCCCAT gen in ACGTATCCACTGACGTCGCAAGCAT human CATGGAACGATTGGTGGCCTCTCCA response CCAGAGTGACCAACCTCACTCAGGT GGCGTTGGGCGTAATTACGGTTTCT ACTACGTGGACACGACTGGAGAGGG CAAGTGTGCACTCTCTGACCAGGTA CCCGACTGCCTGGTGTCGGATTCTG CCGCCGTGTCGTATACAGCAGCGGG GAGTTTGTCTGAAGAGACGCCGAAT TTCATAATTCCGTCAAATCCCTCTG TTACTCCGCCAACGCCCGAGACGGC ACTTCAGTGCACGGCCGACAAGTTC CCCGACTCTTTCGGTGCCTGCGACG TTCAAGCGTGTAAAAGACAGAAGAC GTCCTGCGTTGGCGGACAGATTCAA AGTACTAGCGTCGACTGCACCGCGG ACGAACAAAATGAATGTGGCTCTAA CACTGCG Tx15.b AGTGCCAACGTAACAAGTTCGGAGC MIC4 Micro- (SEQ ID CTGCAAAACTTGATCTCTCTTGTGC neme No 63) GCACTCTGACAATAAGGGATCAAGG protein— GCTCCCACAATAGGCGAGCCAGTGC unknown CAGATGTGTCCCTGGAACAATGTGC as anti- TGCGCAATGCAAGGCTGTTGATGGC gen in TGCACACATTTCACTTATAATGACG human ATTCGAAGATGTGCCATGTGAAGGA response GGGAAAACCCGATTTATACGATCTC ACAGGAGGCAAAACAGCACCGCGCA GTTGCGATAGATCATGCTTCGAACA ACACGTATCGTATGAGGGAGCTCCT GACGTGATGACAGCGATGGTCACGA GCCAGTCAGCGGACTGTCAGGCTGC GTGTGCGGCTGACCCGAGCTGCGAG ATCTTCACTTATAACGAACACGACC AGAAATGTACTTTCAAAGGAAGGGG GTTTTCTGCGTTTAAGGAACGAGGG GTGTTGGGTGTGACTTCCGGGCCGA AACAGTTCTGCGATGAAGGCGGTAA ATTAACT

The clones Tx-2.a e Tx-1.b represent two distinct fragments of the MIC2 gene (Wan et al, 1997, Mol. Biochem. Parasitol. 84: 203-214) and have never been identified as antigens of the human antibody response. Said clones have respectively the amino acid sequences PQDAICSDWSAWSPCSVSCGDGSQIRTRTEVSAPQPGTPTCPDCPA PMGRTCVEQGGLEEIRECSAGVCAVDAGCGVWV (SEQ ID No 64) and PCPINATCGQFEEWSTCSVSCGGGLKTRSRNPWNEDQQHGGLSCE QQHPGGRTETVTCNPQACPVDERPGEWAEWGECSVTCGDGVRER RRGKSLVEAKFGGRTIDQQNEALPEDLKIKNVEYEPCSYPACGASC TYVWSDWNK (SEQ ID No 65) and their use as fragments containing an epitope is covered by the present invention.

The clone Tx-11.b represents a distinct fragment of the M2AP gene (Rabenau et al., 2001, Mol. Microbiol. 41: 537-547) and has never been identified as antigen of the human antibody response. Said clone has the amino acid sequence NEPVALAQLSTFLELVEVPCNSVHVQGVMTPNQMVKVTGAGWDNGVLEFYVTRPTKTGGDTSRSHLASIMCYSK DIDGVPSDKAGKCFLKNFSGEDSSEIDEKEVSLPIKSHNDAFMFVC SSNDGSALQCDVFALDNTNSSDGWKVNTVDLGVSVSPDLAFGLTA DGVKV YASSGLTAINDDPSLGCKAPPHSPPAGEEPSLPSPENS GSATPAEESPSESES (SEQ ID No 66) and its use as fragment containing an epitope is covered by the present invention.

The clone Tx-13.b represents a fragment of the AMA1 gene (Hehl et al., 2000, Infect. Immun. 68:7078-7086). Said clone has the amino acid sequence LRGYRFGVWKKGRCLDYTELTDTVIERVESKAQCWVKTFENDGVASDQPHTYPLTSQASWNDWWPLHQSDQPHSGGVGRNYG FYYVDTTGEGKCALSDQVPDCLVSDSAAVSYTAAGSLSEETPNFIIP SNPSVTPPTPETALQCTADKFPDSFGACDVQACKRQKTSCVGGQIQ STSVDCTADEQNECGSNTA (SEQ ID No 67) and its use as fragment containing an epitope is covered by the present invention.

The clone Tx-15.b represents a fragment of the MIC4 gene (Brecht et al., 2001, J. Biol. Chem. 276:4119-4127). Said clone has the amino acid sequence SANVTSSEPAKLDLSCAHSDNKGSRAPTIGEPVPDVSLEQCAAQCKAVDGCTHFTYNDDSKMCHVKEGKPDLYDLTGGKTAPRS CDRSCFEQHVSYEGAPDVMTAMVTSQSADCQAACAADPSCEIFTY NEHDQKCTFKGRGFSAFKERGVLGVTSGPKQFCDEGGKLT (SEQ ID No 68) and its use as fragment containing an epitope is covered by the present invention.

Expression of DNA Fragments Selected from the Microneme-Library as Fusion Products with GST

Phage clones isolated from the microneme-library were cloned as fusion products with GST protein and expressed in bacterial cells, for the purposes of determining their specificity and selectivity. The procedure described in Examples 1 and 2 was used to produce the fusion proteins.

The following table 10, by way of example, presents the reactivity with negative and positive sera of a number of the clones selected, assayed in the form of fusion proteins: TABLE 10 Reactivity of GST Reactivity of GST fusion protein fusion portein with positive sera with negative sera Name of clone (pos./total neg.) (neg./totale neg.) Tx-2.a 29/30 0/15 Tx-1.b 15/30 0/15 Tx-11.b 23/30 0/15 Tx-13.b 24/30 0/15 Tx-15.b 12/30 0/15 

1. Method for the identification of antigen fragments and/or fragments containing epitopes of Toxoplasma gondii proteins, particularly phase-specific antigen fragments, by means of selection of libraries of cDNA or DNA fragments of specific genes of the parasite with sera of subjects who have been infected, using the phage display technique, characterised in that it uses the vector □KM4.
 2. Antigen fragments and/or fragments containing epitopes of Toxoplasma gondii proteins obtainable with the method according to claim
 1. 3. Antigenic portions of the fragments according to claim
 2. 4. Antigen fragment and/or fragment containing an epitope according to claim 3 with the following amino acid sequence: RRTGCHAFRENCSPGRCIDDASHENGYTCECPTG (SEQ ID NO: 31) YSREVTSKAEESCVEGVEVTLAEKCEKEFGISAS SCKCD LNPIDDMLFETALTANEMMEDITWRPRVDVEFDS (SEQ ID NO: 58) KKKEMIILADLPGLQKDDVTIEVDNGAIVIKGEK TSKEAEKVDDGKTKNILTERVSGYFARRFQLPSN YKPDGISAAMDNGVLRVTIKVEDSGGAKQQISV SGGTGQGLGIGESVDLEMMGNTYRVERPTGNPDL (SEQ ID NO: 26) LKIAIKASDGSYSEVGNVNVEEVIDTMKSMQRDE DIFLRALNKGETVEEAIEDVAQAEGLNSEQTLQL EDAVSAVASVVQDE AALGGLAADQPENHQALAEPVTGVGEAGVSPVNE (SEQ ID NO: 28) AGESYSSATSGVQEATAPGAVLLDAIDAESDKVD NQAEGGERMKKVEEELSLLRRELYDRTDRPG FATAATASDDELMSRIRNSDFFDGQAPVDSLRPT (SEQ ID NO: 29) NAGVDSKGTDDHLTTSMDKASVESQLPRREPLET EPDEQEEVHF PQDAICSDWSAWSPCSVSCGDGSQIRTRTEVSAP (SEQ ID NO: 64) QPGTPTCPDCPAPMGRTCVEQGGLEEIRECSAGV CAVDAGCGVWV PCPINATCGQFEEWSTCSVSCGGGLKTRSRNPWN (SEQ ID NO: 65) EDQQHGGLSCEQQHPGGRTETVTCNPQACPVDER PGEWAEWGECSVTCGDGVRERRRGKSLVEAKFGG RTIDQQNEALPEDLKIKNVEYEPCSYPACGASCT YVWSDWNK LRGYRFGVWKKGRCLDYTELTDTVIERVESKAQC (SEQ ID NO: 67) WVKTFENDGVASDQPHTYPLTSQASWNDWWPLHQ SDQPHSGGVGRNYGFYYVDTTGEGKCALSDQVPD CLVSDSAAVSYTAAGSLSEETPNFIIPSNPSVTP PTPETALQCTADKFPDSFGACDVQACKRQKTSCV GGQIQSTSVDCTADEQNECGSNTA NEPVALAQLSTFLELVEVPCNSVHVQGVMTPNQM (SEQ ID NO: 66) VKVTGAGWDNGVLEFYVTRPTKTGGDTSRSHLAS IMCYSKDIDGVPSDKAGKCFLKNFSGEDSSEIDE KEVSLPIKSHNDAFMFVCSSNDGSALQCDVFALD NTNSSDGWKVNTVDLGVSVSPDLAFGLTADGVKV KKLYASSGLTAINDDPSLGCKAPPHSPPAGEEPS LPSPENSGSATPAEESPSESES GLSQRVPELPEVEPFDEVGTGARRSGSIATLLPQ (SEQ ID NO: 57) DAVLYENSEDVAVPSDSASTPSYFHVESPSASVE AATGAVGEVVPDCEEQQEQGDTTLSDHDFH PSVVNNVARCSYGADSTLGPVKLSAEGPTTMTLV (SEQ ID NO: 34) CGKDGVKVPQDNNQYCSGTTLTGCNEKSFKDILP KLTENPWQGNASSDKGATLTIKKEAFPAESKSVI IGCTGGSPEKHHCTVKLEFAGAAGSAKSA VMASDPPLVANQVVTCPDKKSTAAVILTPTENHF (SEQ ID NO: 33) TLKCPKTALTEPPTLAYSPNRQICPAGTTSSCTS KAVTLSSLIPEAEDSWWTGDSASLDTAGIKLTVP IEKFPVTTQTFVVGCIKGDDAQSCMVTVTVQARA SSVVNNVARCSYGADS APTQSEMKEFQEEIKEGVEETKHEDDPEMTRLMV (SEQ ID NO: 32) TEKQESKNFSKMAKSQSFSTRIEELGGSISFLTE TGVTMIELPKTVSEHDMDQLLH SANVTSSEPAKLDLSCAHSDNKGSRAPTIGEPVP (SEQ ID NO: 68) DVSLEQCAAQCKAVDGCTHFTYNDDSKMCHVKEG KPDLYDLTGGKTAPRSCDRSCFEQHVSYEGAPDV MTAMVTSQSADCQAACAADPSCEIFTYNEHDQKC TFKGRGFSAFKERGVLGVTSGPKQFCDEGGKLT ENPVRPPPPGFHPSVIPNPPYPLGTPAGMPQPEV (SEQ ID NO: 30) P YSSPRIVVLIRYCFFSTYRLTMFAVKHCLLVVAV (SEQ ID NO: 27) GALVNVSVRAAEFSGVVNQGP.


5. Collection of antigen fragments obtainable with the method according to claim
 1. 6. Nucleotide sequence coding for an antigen fragment according to claim
 2. 7. Nucleotide sequence coding for an antigen fragment according to claim 4 selected from the group consisting of: AGGAGGACTGGATGTCATGCCTTCAGGGAGAACT (SEQ ID NO: 22) GCAGCCCTGGTAGATGTATTGATGACGCCTCGCA TGAGAATGGCTACACCTGCGAGTGCCCCACAGGG TACTCACGTGAGGTGACTTCCAAGGCGGAGGAGT CGTGTGTGGAAGGAGTCGAAGTCACGCTGGCTGA GAAATGCGAGAAGGAATTCGGCATCAGCGCGTCA TCCTGCAAATGCGAT TCTTCAGAAAGATGACGTAACCATAGAAGTCGAC (SEQ ID NO: 56) AACGGAGCCATCGTTATCAAAGGAGAGAAGACCT CGAAAGAAGCGGAGAAAGTGGACGATGGCAAAAC AAAGAACATTTTGACTGAGCGAGTGTCCGGTTAT TTTGCGCGCCGGTTCCAGCTCCCGAGTAATTACA AGCCCGACGGAATCAGTGCGGCAATGGACAACGG CGTTCTACGTGTCACGATCAAGGTCGAGGATTCA GGGGGCGCAAAGCAACAAATCAGCGTG AGTGGAGGGACAGGGCAGGGATTAGGAATCGGAG (SEQ ID NO: 17) AATCTGTAGATTTGGAGATGATGGGGAACACGTA TCGTGTGGAGAGACCCACAGGCAACCCGGACTTG CTCAAGATCGCCATTAAAGCTTCAGATGGATCGT ACAGCGAAGTCGGCAATGTTAACGTGGAGGAGGT GATTGATACTATGAAAAGCATGCAGAGGGACGAG GACATTTTCCTTCGTGCGTTGAACAAAGGCGAAA CAGTAGAGGAAGCGATCGAAGACGTGGCTCAAGC AGAAGGGCTTAATTCGGAGCAAACCCTGCAACTG GAAGATGCAGTGAGCGCGGTGGCGTCTGTTGTTC AAGACGAG GCTGCCTTGGGAGGCCTTGCGGCGGATCAGCCTG (SEQ ID NO: 19) AAAATCATCAGGCTCTTGCAGAACCAGTTACGGG TGTGGGGGAAGCAGGAGTGTCCCCCGTCAACGAA GCTGGTGAGTCATACAGTTCTGCAACTTCGGGTG TCCAAGAAGCTACCGCCCCAGGTGCAGTGCTCCT GGACGCAATCGATGCCGAGTCGGATAAGGTGGAC AATCAGGCGGAGGGAGGTGAGCGTATGAAGAAGG TCGAAGAGGAGTTGTCGTTATTGAGGCGGGAATT ATATGATCGCACAGATCGCCCTGGT CAGTTCGCTACCGCGGCCACCGCGTCAGATGACG (SEQ ID NO: 20) AACTGATGAGTCGAATCCGAAATTCTGACTTTTT CGATGGTCAAGCACCCGTTGACAGTCTCAGACCG ACGAACGCCGGTGTCGACTCGAAAGGGACCGACG ATCACCTCACCACCAGCATGGATAAGGCATCTGT AGAGAGTCAGCTTCCGAGAAGAGAGCCATTGGAG ACGGAGCCAGATGAACAAGAAGAAGTTCAT CCCCAGGATGCCATTTGCTCGGATTGGTCCGCAT (SEQ ID NO: 59) GGAGCCCCTGCAGTGTATCCTGCGGTGACGGAAG CCAAATCAGGACGCGAACTGAGGTTTCTGCTCCG CAACCTGGAACACCAACATGTCCGGACTGCCCTG CGCCCATGGGAAGGACTTGCGTGGAACAAGGCGG ACTTGAAGAAATCCGTGAATGCAGTGCGGGGGTA TGTGCTGTTGACGCTGGATGTGGCGTCTGGGTT CCGTGTCCAATTAATGCAACTTGCGGTCAGTTTG (SEQ ID NO: 60) AAGAATGGAGTACATGCTCGGTCTCATGTGGTGG TGGACTGAAAACGAGGTCGAGGAACCCTTGGAAT GAAGACCAACAACATGGAGGACTATCCTGCGAGC AGCAGCATCCTGGTGGGCGGACGGAAACGGTAAC TTGCAATCCTCAAGCGTGTCCTGTGGATGAACGA CCGGGGGAGTGGGCAGAGTGGGGGGAATGTAGTG TCACGTGCGGCGACGGAGTGCGAGAGCGCAGGCG CGGGAAAAGTCTAGTTGAGGCTAAATTCGGCGGA CGCACCATTGATCAGCAGAATGAGGCTCTTCCGG AAGACTTAAAAATCAAAAACGTCGAGTATGAGCC ATGTTCGTATCCTGCTTGTGGAGCTTCCTGCACG TACGTCTGGAGTGACTGGAACAAG CTTCGCGGGTACAGGTTCGGTGTTTGGAAGAAAG (SEQ ID NO: 62) GCCGTTGCCTCGACTACACTGAATTGACCGACAC TGTGATAGAACGTGTTGAGTCAAAGGCACAGTGC TGGGTGAAAACCTTTGAAAACGACGGGGTCGCGA GTGACCAACCCCATACGTATCCACTGACGTCGCA AGCATCATGGAACGATTGGTGGCCTCTCCACCAG AGTGACCAACCTCACTCAGGTGGCGTTGGGCGTA ATTACGGTTTCTACTACGTGGACACGACTGGAGA GGGCAAGTGTGCACTCTCTGACCAGGTACCCGAC TGCCTGGTGTCGGATTCTGCCGCCGTGTCGTATA CAGCAGCGGGGAGTTTGTCTGAAGAGACGCCGAA TTTCATAATTCCGTCAAATCCCTCTGTTACTCCG CCAACGCCCGAGACGGCACTTCAGTGCACGGCCG ACAAGTTCCCCGACTCTTTCGGTGCCTGCGACGT TCAAGCCTGTAAAAGACAGAAGACGTCCTGCGTT GGCGGACAGATTCAAAGTACTAGCGTCGACTGCA CCGCGGACGAACAAAATGAATGTGGCTCTAACAC TGCG AACGAACCGGTGGCCCTAGCTCAGCTCAGCACAT (SEQ ID NO: 61) TCCTCGAGCTCGTCGAGGTGCCATGTAACTCTGT TCATGTTCAGGGGGTGATGACCCCGAATCAAATG GTCAAAGTGACTGGTGCAGGATGGGATAATGGCG TTCTCGAGTTCTATGTCACGAGGCCAACGAAGAC AGGCGGGGACACAAGCCGAAGCCATCTTGCGTCG ATCATGTGTTATTCCAAGGACATTGACGGCGTGC CGTCAGACAAAGCGGGAAAGTGCTTTCTGAAGAA CTTTTCTGGTGAAGACTCGTCGGAAATAGACGAA AAAGAAGTATCTCTACCCATCAAGAGCCACAACG ATGCGTTCATGTTCGTTTGTTCTTCAAATGATGG ATCCGCACTCCAGTGTGATGTTTTCGCCCTTGAT AACACCAACTCTAGCGACGGGTGGAAAGTGAATA CCGTGGATCTTGGCGTCAGCGTTAGTCCGGATTT GGCATTCGGACTCACTGCAGATGGGGTCAAGGTG AAGAAGTTGTACGCAAGCAGCGGCCTGACAGCGA TCAACGACGACCCTTCCTTGGGGTGCAAGGCTCC TCCCCATTCTCCGCCGGCCGGAGAGGAACCGAGT TTGCCGTCGCCTGAAAACAGCGGGTCTGCAACAC CAGCGGAAGAAAGTCCGTCTGAGTCTGAATCT GGATTGAGCCAAAGGGTGCCAGAGCTACCAGAAG (SEQ ID NO: 55) TGGAGCCCTTTGATGAAGTAGGCACGGGAGCTCG ACGGTCCGGGTCCATTGCGACCCTTCTTCCACAA GACGCTGTTTTATATGAGAACTCAGAGGACGTTG CCGTTCCGAGTGATTCAGCATCGACCCCGTCATA CTTTCATGTGGAATCTCCAAGTGCTAGTGTGGAA GCCGCGACTGGCGCTGTGGGAGAGGTGGTGCCGG ACTGTGAAGAACAACAGGAACAGGGTGACACGAC GTTATCCGATCACGATTTCCATTCA CCATCGGTCGTCAATAATGTCGCAAGGTGCTCCT (SEQ ID NO: 25) ACGGTGCAGACAGCACTCTTGGTCCTGTCAAGTT GTCTGCGGAAGGACCCACTACAATGACCCTCGTG TGCGGGAAAGATGGAGTCAAAGTTCCTCAAGACA ACAATCAGTACTGTTCCGGGACGACGCTGACTGG TTGCAACGAGAAATCGTTCAAAGATATTTTGCCA AAATTAACTGAGAACCCGTGGCAGGGTAACGCTT CGAGTGATAAGGGTGCCACGCTAACGATCAAGAA GGAAGCATTTCCAGCCGAGTCAAAAAGCGTCATT ATTGGATGCACAGGGGGATCGCCTGAGAAGCATC ACTGTACCGTGAAACTGGAGTTTGCCGGGGCTGC AGGGTCAGCAAAATCGGCT GTTATGGCATCGGATCCCCCTCTTGTTGCCAATC (SEQ ID NO: 24) AAGTTGTCACCTGCCCAGATAAAAAATCGACAGC CGCGGTCATTCTCACACCGACGGAGAACCACTTC ACTCTCAAGTGCCCTAAAACAGCGCTCACAGAGC CTCCCACTCTTGCGTACTCACCCAACAGGCAAAT CTGCCCAGCGGGTACTACAAGTAGCTGTACATCA AAGGCTGTAACATTGAGCTCCTTGATTCCTGAAG CAGAAGATAGCTGGTGGACGGGGGATTCTGCTAG TCTCGACACGGCAGGCATCAAACTCACAGTTCCA ATCGAGAAGTTCCCCGTGACAACGCAGACGTTTG TGGTCGGTTGCATCAAGGGAGACGACGCACAGAG TTGTATGGTCACGGTGACAGTACAAGCCAGAGCC TCATCGGTCGTCAATAATGTCGCAAGGTGCTCCT ATGGTGCGGACAGC GCACCCACTCAATCTGAAATGAAAGAATTCCAAG (SEQ ID NO: 23) AGGAAATCAAAGAAGGGGTGGAGGAAACAAAGCA TGAAGACGATCCTGAGATGACGCGGCTCATGGTG ACCGAGAAGCAGGAGAGCAAAAATTTCAGCAAGA TGGCGAAATCCCAGAGTTTTAGCACGCGAATCGA AGAGCTCGGGGGATCCATTTCGTTTCTAACTGAA ACGGGGGTCACAATGATCGAGTTGCCCAAAACTG TCAGTGAACATGACATGGACCAACTACTCCAC AGTGCCAACGTAACAAGTTCGGAGCCTGCAAAAC (SEQ ID NO: 63) TTGATCTCTCTTGTGCGCACTCTGACAATAAGGG ATCAAGGGCTCCCACAATAGGCGAGCCAGTGCCA GATGTGTCCCTGGAACAATGTGCTGCGCAATGCA AGGCTGTTGATGGCTGCACACATTTCACTTATAA TGACGATTCGAAGATGTGCCATGTGAAGGAGGGA AAACCCGATTTATACGATCTCACAGGAGGCAAAA CAGCACCGCGCAGTTGCGATAGATCATGCTTCGA ACAACACGTATCGTATGAGGGAGCTCCTGACGTG ATGACAGCGATGGTCACGAGCCAGTCAGCGGACT GTCAGGCTGCGTGTGCGGCTGACCCGAGCTGCGA GATCTTCACTTATAACGAACACGACCAGAAATGT ACTTTCAAAGGAAGGGGGTTTTCTGCGTTTAAGG AACGAGGGGTGTTGGGTGTGACTTCCGGGCCGAA ACAGTTCTGCGATGAAGGCGGTAAATTAACT GAGAACCCGGTGAGACCGCCTCCTCCCGGTTTCC (SEQ ID NO: 21) ATCCAAGCGTTATTCCCAATCCCCCGTACCCGCT GGGCACTCCAGCGGGCATGCCACAGCCAGAGGTT CC TACTCTTCACCACGAATAGTTGTTTTGATTAGAT (SEQ ID NO: 18) ATTGCTTCTTCTCCACATATCGCCTCACAATGTT CGCCGTAAAACATTGTTTGCTGGTTGTTGCCGTT GGCGCCCTGGTCAACGTCTCGGTGAGGGCTGCCG AGTTTTCCGGAGTTGTTAACCAGGGACCT


8. Nucleotide sequences that hybridise with the sequences according to claim 6, and/or with the sequences coding for the sequences according to claim 4, under stringent hybridisation conditions.
 9. Nucleotide sequences that hybridise with the sequences coding for the antigen fragments according to claim 2 under stringent hybridisation conditions. 10-12. (canceled)
 13. Specific ligand for an epitope according to claim
 2. 14. Anti-epitope antibody raised against an epitope according to claim
 2. 15. Ligands for the collection according to claim
 5. 16. Anti-collection antibody raised against a collection according to claim
 5. 17-18. (canceled)
 19. Method for the diagnosis of Toxoplasma gondii infection, comprising the selection of sera of subjects affected by or suspected of being affected by said infection with the antigen fragments of claim 1 and/or with at least one ligand of said antigen, and/or at least one antibody to the same. 20-23. (canceled)
 24. Diagnostic kit for the diagnosis of Toxoplasma gondii infection, containing at least one antigen fragment according to claim
 2. 25. Kit for the diagnosis of an acute and/or chronic Toxoplasma gondii infection, containing at least one antigen fragment according to claim
 4. 26. Pharmaceutical composition, particularly in the form of a vaccine, containing at least one antigen fragment according to claim
 2. 27. Pharmaceutical composition, particularly in the form of a vaccine, containing at least one sequence according to claim
 6. 28. Composition according to claim 26 suitable for human and/or veterinary use. 